This story takes place during the episode "imagination Manifestation"

The strange magenta dress Rarity magically placed on Rainbow Dash seemed to have some strange possession magic left over from the mysterious book Spike found. Not only does she now want to wear it the dress is making her act more like a fashion model and the longer she has it on the more her personality is changing. While Twilight is finding any way to help Rainbow Dash get back to normal Rarity is enjoying the new Rainbow Dash too much and this could be making things worse.

Cover Art: I don't know who made this the artist was from Bronycon last year